Real-time measurements of crystallization processes in visco-elastic polymeric photonic crystals

We present a study of the dynamic shear-ordering of viscoelastic photonic crystals, based on core-shell polymeric composite particles. Using an adapted shear-cell arrangement, the crystalline ordering of the material under conditions of oscillatory shear are interrogated in real-time, through both video imaging and from the optical transmission spectra of the cell. In order to gain a deeper understanding of the macroscopic influences of shear on the crystallization process in this solvent free system, the development of bulk ordering is studied as a function of the key parameters including duty cycle and shear-strain magnitude. In particular, optimal ordering is observed from a pre-randomized sample at shear strains of around 160%, for 1Hz oscillations. This ordering reaches completion over timescales of order 10 seconds. These observations suggest significant local strains are needed to drive nanoparticles through energy barriers, and that local creep is needed to break temporal symmetry in such high viscosity nano-assemblies. Crystal shear-melting effects are also characterized under conditions of constant shear rate. These quantitative experiments aim to stimulate the development of new theoretical models which can deal with the strong local particle interactions in this system.